The present invention relates to a radio telephone system and, more particularly, to connection control between a base station and a plurality of mobile stations in the radio telephone system.
As shown in FIG. 1, a conventional radio telephone system comprises a base station 1 connected to a telephone exchanger 3 through a subscriber line 110, and a plurality of mobile stations 201 connected to the base station 1 through a radio channel.
The base station 1 comprises a controller 101 having a microprocessor as a main component, a transmitter/receiver 102 for exchanging signals with the mobile stations 201 through a radio channel, and a hybrid circuit 103 including an interface circuit for interfacing data with the subscriber line 110 and a communication network. The radio channel comprises a control channel and communication channels. The controller 101 has a memory 106 and timers 107 and 108, and controls the transmitter/receiver 102, and the hybrid circuit 103 to control connection and communication between the base station 1 and the mobile stations 201 through a radio channel.
In a multi-access radio telephone system, a plurality of radio telephone sets having a given radio frequency band are simultaneously used in a relatively narrow area. A radio wave emitted from one radio telephone set can reach all other radio telephone sets. The radio telephone sets use common communication channels the number of which is smaller than that of mobile stations (e.g., 3 common channels). When a given radio telephone set is off-hooked using communication channel #1, the remaining radio telephone sets can use one of communication channels #2 and #3 after they check whether the channels #2 and #3 are vacant.
Stand-by and connection control procedures in the conventional radio telephone sets in the multi-access radio telephone system will be described with reference to FIGS. 2 and 3.
FIG. 2 is a flow chart of the stand-by control procedure of the base station 1. Referring to FIG. 2, the controller 101 controls the transmitter/receiver 102 to switch a channel subjected to radio reception to a control channel (step 500). The timer 107 is set for, e.g., one second (step 501). The controller 101 repeatedly checks in step 502 whether a calling signal is received from the mobile station 201 and in step 503 whether a calling signal from the telephone exchanger 3 is detected until the time (one second) set in the timer 107 has elapsed (step 504).
When the calling signal transmitted from the mobile station 201 to the base station 1 (referred to as calling signal from the mobile station) is detected in step 502, the flow advances to processing of the calling signal from the mobile station (a detailed description thereof will be omitted) to connect this mobile station 201 to the telephone exchanger 3 (step 508).
When the calling signal transmitted from the telephone exchanger 3 to the base station 1 (referred to as calling signal from the telephone exchanger) is detected in step 503, processing of the calling signal from the telephone exchanger 3 (step 509) is performed for connecting the called mobile station 201 to the telephone exchanger 3 (this processing will be described in detail later).
When the lapse of one second set in the timer 107 is detected in step 504, the controller 101 controls the transmitter/receiver 102 to switch the radio channel from the control channel to one arbitrary communication channel (step 505). The controller 101 determines in step 506 whether the communication channel is busy. If YES in step 506, the flow returns to step 500. However, if NO in step 506, the communication channel which is not busy is memorized in the memory 106 (step 507). Thereafter, the flow returns to step 500, and the above stand-by control procedure is repeated.
FIGS. 3A and 3B are flow charts of processing of the calling signal from the telephone exchanger 3 (step 509 in FIG. 2). Referring to FIGS. 3A and 3B, in the base station 1 in which the calling signal from the telephone exchanger 3 is detected by forming a current loop with the subscriber line 110, the controller 101 in the base station 1 determines in step 600 whether other radio telephone sets use the control channel. If the control channel is not used, a predetermined time (e.g., 4 seconds) is set in the timer 108 in step 601. The controller 101 transmits the calling signal from the telephone exchanger to all the mobile stations 201 through the control channel (step 602). The controller 101 determines in step 603 whether the responses to the calling signal from the telephone exchanger have been received from the mobile stations 201. When each mobile station 201 detects reception of the calling signal from the telephone exchanger in step 650, the response is transmitted to the base station 1 in step 651. The operation of the base station 1 in step 603 continues until the lapse of four seconds preset in the timer 108 is detected in step 604. When the responses are detected in step 603, or when the lapse of four seconds preset in the timer 108 is detected in step 604, the controller 101 in the base station 1 controls the transmitter/receiver 102 to switch the channel from the control channel to a given communication channel (step 605).
In this case, since a communication channel assignment signal is transmitted from the base station 1 to the mobile stations 201, the channel of each mobile station 201 is switched to the assigned communication channel (step 652).
Subsequently, the base station 1 receives a ringing signal of a 16-Hz intermittent signal from the telephone exchanger 3 and transmits a bell signal through the communication channel as long as the ringing signal is received (steps 606 and 607). The controller 101 then waits for an off-hook signal (step 608).
In each mobile station 201, the bell rings when the bell signal is received (steps 653 to 655), thereby causing a user to off-hook the mobile telephone set (step 656). When the mobile station 201 detects the off-hook state (step 656), the mobile station 201 transmits an off-hook signal to the base station 1 (step 657). The off-hook signal is received by the base station 1 (step 608). Thereafter, exchange of the communication signal using the assigned communication channel is performed between the base station 1 and the mobile station 201.
In the conventional multi-access radio telephone system described above, the following problems are presented.
In the stand-by mode of the base station 1, the controller 101 controls the transmitter/receiver 102 to switch the control and communication channels every predetermined time interval, thereby monitoring reception of the calling signal from the mobile station 201 and monitoring of the vacant communication channel. Therefore, when the calling signal is transmitted from the mobile station 201 to the base station 1 while the base station 1 monitors the vacant communication channel (steps 505 to 507 in FIG. 2), the base station 1 cannot immediately detect the calling signal from the mobile station until the channel is switched to the control channel (steps 500 to 502 in FIG. 2).
Assume that a call is made from the telephone exchanger 3 when one, e.g., 201(A) of the plurality of mobile stations 201 is kept deenergized, that is, turns off its power switch. The mobile station 201(A) can detect the call by ringing of the bells of other mobile stations, i.e., the mobile station 201(B) and the like. Since the base station 1 and other mobile stations such as the station 201(B) are switched to a communication channel, the mobile station 201(A) cannot be connected to the base station 1. Therefore, the mobile station 201(A) cannot participate in communication in response to the call from the telephone exchanger 3.